xvin THE CLASSIFICATION OF THE ELEMENTS 495 



elements, and showed that these might be identical in different 

 families ; thus there was a common difference between the 

 equivalents in the families F, Cl, Br, I and N, P, As, Sb, as well 

 as between those of Mg, Ca, Sr, Ba, Pb and O, S, Se, Te, Os. 



Newlands, in 1864, using Cannizzaro's atomic weights, 

 arranged Dobereiner's triads and Dumas's families in a single 

 table. In 1865, after tabulating the elements in the order of the 

 new atomic weights, he found that similar elements occurred, 

 as in music, at intervals of 7 or 14 places. This law of octaves 

 failed to attract attention until similar schemes were put 

 forward by Mendele"eff in 1869 and in 1871 and by Lothar 

 Meyer in 1869 to illustrate the law of periodicity, which states 

 that " The properties of the elements are periodic functions 

 of their atomic weight? In Mendeleeff's classification, New- 

 lands's octaves appear again as 12 series or short periods, the 

 first containing hydrogen only, whilst the second contains the 

 7 typical elements, Li, Be, B, C, N, O, F, of Groups I to VII. 

 A Group VIII was added to take the triads of transition-elements, 

 which bridge the gap between the even series and the odd series, 

 4 to 5, 6 to 7, and 10 to n ; each of these triads, with the 

 octaves on either side, constituted a long period of 7 + 3 + 7 = 17 

 elements. A " Group o " is now added to take the rare-gases 

 of the helium series. The main periods, as now recognised, 

 are : 



Period I. ( = series i) Hydrogen. 



Period II and III. ( = series 2 and 3) Eight elements each. 



Periods IV and V. ( = series 4 to 7) Eighteen elements each. 



Period VI. ( = series 8 to n) Thirty-two elements. 



Period VII. (= series 12 etc.) Radioactive elements. 



The sixth period is extended to 32 elements by a cluster of 

 some 15 rare-earth elements, crowded together in the boron- 

 group. The end of the sixth period and the fragmentary begin- 

 ning of the seventh period contain 7 elements of known atomic 

 weight from lead to uranium, but the whole space is crowded 

 with some 40 transient radio-elements, four of these occupying 

 well-defined places at the beginning of the seventh period ; 

 others are clustered together in small swarms of isotopic elements 

 having similar chemical properties and nearly equal atomic 

 weights. 



